Hydrogenation of Esters to Alcohols Catalyzed by Defined Manganese Pincer Complexes

Article abstract of DOI:10.1002/anie.201607233

The first manganese-catalyzed hydrogenation of esters to alcohols has been developed. The combination of Mn(CO)₅Br with [HN(CH₂CH₂P(Et)₂)₂] leads to a mixture of cationic and neutral Mn PNP pincer complexes, which enable the reduction of various ester substrates, including aromatic and aliphatic esters as well as diesters and lactones. Notably, related pincer complexes with isopropyl or cyclohexyl substituents showed very low activity.

Full text of DOI:10.1002/anie.201607233

Angewandte
Communications
Chemie
International Edition: DOI: 10.1002/anie.201607233
German Edition: DOI: 10.1002/ange.201607233
Hydrogenation Very Important Paper
Hydrogenation of Esters to Alcohols Catalyzed by Defined Manganese
Pincer Complexes
Saravanakumar Elangovan+, Marcel Garbe+, Haijun Jiao, Anke Spannenberg, Kathrin Junge,
and Matthias Beller*
Abstract: The first manganese-catalyzed hydrogenation of
esters to alcohols has been developed. The combination of
Mn(CO)5Br with [HN(CH2CH2P(Et)2)2] leads to a mixture of
cationic and neutral Mn PNP pincer complexes, which enable
the reduction of various ester substrates, including aromatic
and aliphatic esters as well as diesters and lactones. Notably,
related pincer complexes with isopropyl or cyclohexyl sub-
stituents showed very low activity.
plexes[13] have been published for effective ester hydrogena-
tion.
After iron and titanium, manganese is the most abundant
transition metal in the Earthꢀs crust. Moreover, most com-
pounds of this latter element show low toxicity, which
currently makes it a highly attractive aspirant for the design
of new catalysts.[14] In general, manganese catalysts are well
known for oxidation reactions whereas reductive transforma-
tions were basically unknown until very recently.[15,16] Thus we
were very surprised when we discovered that catalytic
hydrogenations of aldehydes, ketones, and nitriles can be
conducted in the presence of manganese pincer complex
1 whereas Kempe et al. were able to reduce ketones and
aldehydes under milder conditions by using a PN5P ligand.[17]
Inspired by these recent developments, we also became
interested in manganese-catalyzed ester hydrogenations. To
the best of our knowledge, no manganese catalyst has been
described for such reductions. Herein, we present the first
example of an efficient and selective method for the hydro-
genation of various esters into the corresponding alcohols in
the presence of molecularly defined manganese complexes
(Figure 1).
T
he catalytic hydrogenation of esters to the corresponding
alcohols is an important basic transformation for organic
synthesis, which is also applied in industry for producing
flavors, fragrances, and other fine-chemical intermediates as
well as monomers for polyesters.[1] Compared to classic
stoichiometric reductions, these reactions are advantageous
with respect to costs and waste formation.[2] Although many
hydrogenations of aldehydes and ketones have been reported,
the reduction of carboxylic acids and their esters using H2 is
a more challenging task owing to the lower electrophilicity of
the carbonyl carbon atom.[3]
In this respect, the development of more active well-
defined homogeneous catalysts for ester hydrogenation is
interesting.[4] Notably, in the past decade, several groups from
industry (e.g., Firmenich) and academia reported defined
metal complexes based on ruthenium,[5] osmium,[6] and
iridium[7] for this transformation. However, these expensive
and potentially toxic noble metals should ideally be replaced
by earth-abundant, inexpensive, and environmentally more
benign metals, especially iron and manganese.[8] Whereas in
recent years, major developments have been achieved with
iron and cobalt complexes both for hydrogenation and
dehydrogenation reactions, manganese is much less
explored.[9] For example, in 2014, Milstein and co-workers
reported the first iron-catalyzed hydrogenation of activated
esters to the corresponding alcohols.[10] Furthermore, our
group and Guan and co-workers independently described
hydrogenation reactions of non-activated esters with PNP
iron pincer complexes.[11] Since then, an improved second-
generation iron pincer complex[12] and different cobalt com-
Figure 1. Manganese pincer complexes used in this study.
Initial experiments were performed using methyl ben-
zoate (6a) as a benchmark substrate in the presence of
2 mol% catalyst at 30 bar H2 and 1008C. Unfortunately,
complex 1, which showed high activity in the hydrogenation
of ketones, afforded only very low yields of the desired
alcohol (Table 1, entry 1). Similarly, the cyclohexyl-substi-
tuted manganese pincer complex 2 proved to be not suitable
(entry 2). To improve the catalyst activity, we focused on the
synthesis of less hindered manganese complexes. Therefore,
reactions of Mn(CO)5Br with the Et2PNP pincer ligand were
performed. However, in toluene at 1008C, a mixture of the di-
and tricarbonyl manganese complexes 3 and 4 was obtained
(3: 64%; 4: 18%), which could be isolated and characterized
by spectroscopic methods (see the Supporting Information).
Interestingly, complex 4 can be formed in higher yield (72%)
when 3 is heated to reflux in toluene for additional 16 h. X-ray
[*] S. Elangovan,[+] M. Garbe,[+] Dr. H. Jiao, Dr. A. Spannenberg,
Dr. K. Junge, Prof. Dr. M. Beller
Leibniz-Institut fꢀr Katalyse e. V. an der Universitꢁt Rostock
Albert-Einstein Strasse 29a, Rostock 18059 (Germany)
E-mail: Matthias.Beller@catalysis.de
[+] These authors contributed equally to this work.
Supporting information and the ORCID identification number(s) for
the author(s) of this article can be found under:
http://dx.doi.org/10.1002/anie.201607233.
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+
+
À
Table 1: Manganese-catalyzed hydrogenation of methyl benzoate (6a):
trans from the N H deprotonation of 3 -cis and 3 -trans was
computed. Again, the trans complex is more stable than the
cis isomer by 10.1 kcalmolÀ1. Furthermore, the energies of
CO dissociation from 5-trans and 5-cis resulting in 3b were
computed. CO dissociation from 5-trans is endergonic by
2.4 kcalmolÀ1 whereas CO dissociation from 5-cis is exergonic
by 7.7 kcalmolÀ1.
Optimization of the reaction conditions.[a]
Entry
Catalyst
Solvent
p [bar]
T [8C]
Yield[b] [%]
1
2
1
2
1
3
3
3
3
3
4
toluene
toluene
toluene
toluene
1,4-dioxane
1,4-dioxane
1,4-dioxane
1,4-dioxane
1,4-dioxane
30
30
80
30
30
10
30
30
30
100
100
120
100
100
100
110
80
6
2
Catalytic tests using 3 in the benchmark reaction revealed
a significantly enhanced activity and gave benzyl alcohol in
82% yield (Table 1, entry 4). Even better results were
obtained in 1,4-dioxane (93%; entry 5). Interestingly, even
at low pressure (10 bar H2), the desired product was obtained
(51%, entry 6). Slightly increasing the temperature to 1108C
led to nearly quantitative yield (97%, entry 7). As expected, 4
showed similar activity to that of 3 under the optimized
reaction conditions (entries 7 and 9), suggesting that both
precursors 3 and 4 form the same active species 3b.
3[c]
4
38
82
93
51
97
46
97
5
6
7
8
9
110
[a] General reaction conditions: 6a (0.5 mmol), 1–4 (2 mol%), solvent
(1 mL), t-BuOK (10 mol%), 24 h, 80–1108C, 10–30 bar H2. [b] Yield
determined by GC analysis using hexadecane as an internal standard.
[c] 1 (3 mol%).
Furthermore,
a yield/time analysis showed comparable
behavior for complexes 3 and 4 (see the Supporting Informa-
tion), which supports this hypothesis.
Next, the general applicability of these manganese
catalysts was tested for the reduction of various esters,
including aromatic and aliphatic esters as well as diesters and
lactones (2 mol% catalyst loading, 10 mol% t-BuOK, 30 bar
H2, 1108C, 1,4-dioxane). Testing selected substrates with
complexes 3 and 4 confirmed that these complexes gave the
corresponding products in equal yields.
analysis of tricarbonyl complex 3 revealed that the Mn center
has a distorted octahedral coordination sphere, where both
P atoms and the N atom as well as the three CO ligands are
located cis to each other (Figure 2). This configuration with
Esters containing electron-donating and electron-with-
drawing substituents were hydrogenated to the corresponding
alcohols with moderate to good yields (Table 2, entries 1–7).
Furthermore, sterically hindered (6h), electron-rich (6i), and
electron-poor (6j) esters as well as heteroaromatic substrates
(6l and 6m) were smoothly reduced, and the corresponding
products were isolated in good yields (78–95%).
Next, we examined the hydrogenation of benzylic and
aliphatic esters under the optimized reaction conditions. In all
Figure 2. Molecular structures of complexes 3 (left) and 4 (right). Only
the cation of 3 is shown. Thermal ellipsoids set at 30% probability.
Hydrogen atoms except for those attached to nitrogen omitted for
clarity.
Table 2: Manganese-catalyzed hydrogenation of aromatic esters.[a]
the two P atoms in cis position is surprising as in PNP pincer
complexes, such as 1, 2, and 4 as well as related Fe, Ru, Os,
and Ir pincer complexes, the two P atoms usually adopt a trans
orientation.
Entry
Substrate
R1
R2
Yield[b] 7 [%]
To understand this unusual coordination mode, the
relative energies of the cationic tricarbonyl complexes with
two P ligands in cis (3+-cis) and trans (3+-trans) position were
computed (Figure 3). 3+-trans is more stable than 3+-cis by
7.9 kcalmolÀ1, indicating that 3+-cis should be the kinetic
product. Furthermore, the relative stability of 5-cis and 5-
1
2
3
4
5
6
7
8
6a
6b
6c
6d
6e
6 f
6g
6h
6i
C6H5
C6H5
p-MeC6H4
p-MeOC6H4
p-CF3C6H4
p-ClC6H4
Me
t-Bu
Me
Me
Me
Me
Me
Me
Me
Me
Me
Me
Me
97[c]
98[c]
86
95
75
91
89
95
89
p-FC6H4
2-MeOC6H4
3,4,5-(MeO)3C6H2
2,5-Cl2C6H3
2-naphthyl
2-furyl
9
10
11
12
13
6j
6k
6l
87
86
87[c]
78[c]
6m
3-pyridinyl
[a] Substrate (1 mmol), 3 (2 mol%), 1,4-dioxane (2 mL), 24 h, 1108C,
30 bar H2. [b] Yields of isolated products are given. [c] Determined by GC
analysis using hexadecane as an internal standard.
Figure 3. Calculated structures of different manganese complexes.
E=P(Et)2.
2
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Table 3: Manganese-catalyzed hydrogenation of benzylic and aliphatic
esters as well as diesters and lactones.[a]
Entry
Ester
Alcohol
Conv. (yield) [%]
Figure 4. Hydrogenation of bio-based carboxylic esters. General reac-
tion conditions: substrate (1 mmol), 3 (2 mol%), 1,4-dioxane (2 mL),
24 h, 1108C, 30 bar H2. The conversions were determined by GC
analysis using hexadecane as an internal standard. Yields of isolated
products are given in parentheses. [a] 3 (3 mol%), 48 h, 1208C. [b] The
GC yield was determined with hexadecane as an internal standard.
1
2
3
4
9a R=H
9b R=Cl
9c R=F
>99 (79)
>99 (87)
>99 (93)
>99 (95)
9d R=t-Bu
3-cyclohexene-1-carboxylate (9e) or bio-based methyl oleate
10a (Figure 4) was used as the substrate, whereas the
conjugated ester methyl cinnamate (9 f) produced the satu-
rated alcohol (93%).
5[b,d]
9e
9 f
98 (83)
The versatility of complex 3 was further demonstrated for
the reduction of lactones and diesters to diols (Table 3,
entries 9–14). For example, the flavor and fragrance agent
g-octalactone was converted into the corresponding diol in
good yield. Notably, the clean reduction of bio-mass derived
g-valerolactone (10b; GVL),[18] which is of actual interest for
biorefinery, was successfully achieved to give 1,5-pentanediol
in excellent yield (Figure 4). Finally, aromatic and aliphatic
diesters were reduced to the corresponding diols in moderate
to good yields (Table 3, entries 12 and 14).
To elucidate the reaction mechanism and to understand
the different performance of catalysts 1a, 2a, and 3a, which
can be obtained from the corresponding precatalysts 1, 2, and
3, B3PW91 DFT computations were carried out (see the
Supporting Information). The formation of 3a was proposed
based on the fact that the cationic ethyl complex 3 and the
neutral ethyl complex 4 show the same catalytic performance.
This assumption was supported by NMR experiments where
the active amido species 3b was observed when precatalysts 3
and 4 were treated with base (see the Supporting Informa-
tion).
6
>99 (93)
7[c]
9g
97 (88)
8[d]
9h
9i
92 (71)
9
>99 (82)
10
9j
>79 (57)
>99 (88)
11[d]
9k
Modelling studies showed that the hydride complexes 1a–
3a are very stable towards CO dissociation. Furthermore,
a well-balanced equilibrium is established for the concerted
H2 elimination from 1a–3a to the respective amido complexes
1b–3b. For all complexes, the barrier of H2 elimination is
around 20 kcalmolÀ1, and the reactions are slightly exergonic
by 0.2–1.5 kcalmolÀ1.
On the base of our computations, the following outer-
sphere mechanism was postulated (Scheme 1). Methyl ben-
zoate (6a) is reduced in two cycles via the corresponding
hemiacetal 7c. A stepwise process was identified for the first
cycle (6a to 7c). The hydrogen atoms are transferred in
a stepwise process as a hydride from the manganese center
and as a proton from the nitrogen ligand in 1a (2a or 3a),
resulting in amido complex 1b (2b or 3b). In the first step,
12[c,d]
9l
>93 (66)
>99 (95)
13
9m
14[c]
9n
>99 (58)
[a] Substrate (1 mmol), 3 (2 mol%), 1,4-dioxane (2 mL), 24 h, 1108C,
30 bar H2. The conversions were determined by GC using hexadecane as
an internal standard. Yields of isolated products are given in paren-
theses. [b] 48 h. [c] 3 (3 mol%), 48 h, 1208C. [d] The GC yield was
determined using hexadecane as an internal standard.
À
Mn H···C transfer results in an intermediate, and the second-
À
step N H···O transfer results in 7c. Furthermore, it was found
that for all catalysts, the first step has a higher barrier than the
second step (32.7 vs. 30.1, 34.1 vs. 27.9, and 31.8 vs.
25.8 kcalmolÀ1, respectively, for 1a, 2a, and 3a).
cases, selective reduction to the corresponding alcohol
proceeded in good to very good yields (Table 3, 9a–9h).
Interestingly, the isolated double bond remained intact when
Next, hemiacetal 7c dissociates to give benzaldehyde 7b
and methanol while the amido complex is regenerated by
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Scheme 1. Proposed mechanism for the manganese-catalyzed ester
hydrogenation using 6a as a representative substrate.
addition of H2. In the second cycle, benzaldehyde 7b is
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,
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respectively, for 1a, 2a, and 3a). Based on the energy barriers
of both cycles, we propose the initial reduction of the ester to
be rate-determining. Comparing the different catalysts for
this first hydrogenation cycle, 3a/3b has the lowest barrier
followed by 1a/1b and 2a/2b, which is in agreement with the
observed catalytic activity (3 > 1 > 2).
In conclusion, we have described the first hydrogenation
of esters to alcohols that is catalyzed by molecularly defined
manganese complexes. This catalytic system enabled the
effective and selective hydrogenation of various aromatic and
aliphatic esters as well as diesters and lactones. Experimental
evidence and DFT calculations showed that the reaction
proceeds through an outer-sphere mechanism.
Keywords: alcohols · esters · homogeneous catalysis ·
hydrogenation · manganese
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b) J. M. Tukacs, D. Kirꢂly, A. Strꢂdi, G. Novodarszki, Z. Eke, G.
Received: July 26, 2016
Published online: && &&, &&&&
Angew. Chem. Int. Ed. 2016, 55, 1 – 6
ꢀ 2016 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.angewandte.org
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These are not the final page numbers!

Angewandte
Communications
Chemie
Communications
Hydrogenation
S. Elangovan, M. Garbe, H. Jiao,
A. Spannenberg, K. Junge,
M. Beller*
&&&& — &&&&
Hydrogenation of Esters to Alcohols
Catalyzed by Defined Manganese Pincer
Complexes
A manganese-catalyzed hydrogenation of
esters to alcohols has been developed.
The combination of Mn(CO)5Br with
[HN(CH2CH2P(Et)2)2] led to a cationic
and a neutral Mn PNP pincer complex,
which both enable the reduction of vari-
ous aromatic and aliphatic esters as well
as diesters and lactones.
6
www.angewandte.org
ꢀ 2016 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Angew. Chem. Int. Ed. 2016, 55, 1 – 6
These are not the final page numbers!